Current sources are used in many types of electronic equipment and circuits to provide a constant supply of current. For example, current sources are used to power constant voltage generators, analog circuits, microprocessors, oscillators and analog/digital or digital/analog converters. A complex electronic circuit may be equipped with a plurality of current sources, each intended to power various parts or components of the circuit.
To illustrate the state of the prior art, FIG. 1 attached shows a current source equipped with a known type of starting aid apparatus. The current source is shown as general reference 9 in the diagram of FIG. 1. The current source comprises a first branch 10 connected between a power supply terminal 14 and a ground terminal 16. A first transistor 24 called the mirror transistor is connected in series with a second bipolar npn transistor 26 and a resistor 28 called the emitter resistor. The emitter resistor connects the emitter of bipolar transistor 26a to the ground terminal 16.
The first branch 10 is also called the pilot branch of the current source. A second branch 11 of the current source is connected in parallel with first branch 10 between the positive power supply terminal 14 and the ground terminal. The second branch comprises a first transistor 25 called the mirror transistor and a second bipolar npn transistor 27 both connected in series. The bases of bipolar transistors 26 and 27 of the first and second branches are connected to one another and to the collector of transistor 27 of the second branch.
The bipolar transistors of the two branches of the current source have different emitter surfaces. Transistors 26 and 27 thus have a voltage difference between their bases and their emitters. In the diagram of FIG. 1 bipolar transistor 26 of the first branch is considered to have a larger emitter surface than that of bipolar transistor 27 of the second branch. The difference between base-emitter voltages V.sub.BE 26 and V.sub.BE 27 of bipolar transistors 26 and 27 of the first and second branches, 10, 11, respectively, is noted .delta.V.sub.BE =V.sub.BE 26-V.sub.BE 27. This voltage difference is relayed to the terminals of the emitter resistor 28 that carries a current I.sub.10 such that ##EQU1## where R is the value of emitter resistor 28.
As a first approximation it can be considered that current I.sub. 10, which corresponds to the emitter current of bipolar transistor 26 of the first branch, also corresponds to its collector current. The current I.sub.10 is, therefore, the current of the first branch 10 of the current source.
Mirror transistors 24 and 25 define a current mirror enabling current I.sub.10 flowing in the first pilot branch 10 to be copied to the second branch 11. Designating the current of the second branch 11 as I.sub.11, i.e., more or less the emitter current of transistor 27, it can be verified that I.sub.11 .apprxeq.I.sub.10.
On power-up of the current source it can operate in two different modes. In the first operating mode it can be verified that I.sub.11 =I.sub.10 =0 Amp. This operating mode is clearly undesirable since no current is flowing through the current source. In the second operating mode it can be verified that ##EQU2## as stated above.
To ensure that the source operates in the second mode, a start-up current is injected into one of the branches immediately after power up of the source. This current creates a slight imbalance between the two branches of the source and avoids the first operating mode where I.sub.11 =I.sub.10 =0.
In the rest of this description the term "transient operating state" is understood to mean the operating state of the source immediately after power-up, before the current flowing through branches 10 and 11 has reached its nominal value. The start-up current is injected during this transient operating state.
By "stationary operating state" is understood an operating state reached at the end of the transient state during which the current flowing in the branches of the current source reaches its nominal value, i.e., ##EQU3##
In the stationary state the start-up current can and should be interrupted to avoid excessively high consumption of electricity.
Reference 30 of FIG. 1 is a start-up aid circuit for current source 10. The start-up aid circuit also comprises a pilot branch 32 with a first transistor 34. A second transistor 36 that forms a current mirror with first transistor 34 is provided to copy the current in pilot branch 32 of start-up aid circuit to second branch 11 of current source 9.
The first transistor 34 of the pilot branch is connected between power supply terminal 14 and ground terminal 16 in series with the channel of a field-effect transistor 38 called the resistance transistor used here as a high-rating resistor. The gate of this transistor is connected to power supply terminal 14. It should also be noted that the gate of first transistor 34 is connected to its drain.
The gates of first and second transistor 34, 36 of the start-up aid circuit are connected to one another and to power supply terminal 14 (positive) via a transistor 40 called the blocking transistor. The gate of the blocking transistor is connected to the gates of first transistors 10 and 11 of current source 19, again in a mirror-type assembly.
On power-up of the current source and start-up aid circuit, no current initially flows through branches 10 and 11 of the current source. Blocking transistor 40, driven by the current source, is in the closed state in which no current passes through it.
The first transistor 34 of the start-up aid circuit is designed to conduct when blocking transistor 40 is in the closed state. In this state, a current passes through pilot branch 32 and is copied by means of second transistor 36 to current source 9 as the start-up aid current. When the current source reaches the stationary state, a current flows through branches 10 and 11.
Blocking transistor 40, connected as a current mirror with first transistors 24 and 25 of the current source, starts conducting and takes the gates of first and second transistors 34, 36, to more or less the potential V.sub.cc of the power supply voltage. First and second transistors 34, 36 of the start-up aid circuit are then in the closed state and the start-up aid current is interrupted.
When first transistor 34 is closed the current flowing in pilot branch 32 is interrupted. However, because blocking transistor 40 is conducting and the gate of first transistor 34 is connected to its drain, a current flows towards and through transistor 38, known as the resistance transistor. This current, which is permanent when the source is in stationary state, contributes to the total electrical consumption of the circuit. To minimize this consumption the resistance of the channel of resistance transistor 38 must be increased.
In one embodiment of the circuit shown in FIG. 1 the channel of resistance transistor 38 is 5 .mu.m wide and 4,000 .mu.m long and has a resistance of 20 M.OMEGA.. It will be immediately apparent that this type of component is particularly bulky, especially when required for use in the creation of integrated circuit devices.
When an electronic apparatus comprises a plurality of components requiring independent current sources, multiplying start-up aid circuits for these sources is a problem, particularly in integrated versions of the apparatus. This problem is compounded by the large size of the resistance transistor described above. On this subject it has been noted that reducing the size, and, therefore, the resistance of the channel of this transistor is at the cost of an increase in the current consumed. This again is a problem when creating integrated chip apparatuses and devices.